Speed sensing plays an important role in monitoring, and thus controlling, machine operations. An accurate and reliable speed sensor is critical. Over the years numerous speed-sensing techniques and devices have been developed. Mechanical speedometers, electro-mechanical speed sensors, magnetic speed sensors, and optical speed sensors are just a few examples. Most popular speed sensing systems often include a single sensor, an electronic control unit, and a target whose speed relative to the single sensor is measured.
Depending upon the type of speed being measured, i.e., linear or angular speed, and on the sensor technology that is employed, a target may be constructed in a variety of ways and may take many different forms. Conventionally, speed sensing targets have been made from marked bars and toothed wheels, from multi-polar magnetic-strips and magnetic-rings, and from linear and angular bar-encoders. As the target moves relative to the sensor, a conventional sensor output signal takes the form of a series of pulses, with the pulse frequency being proportional to the target wheel speed.
The resolution or accuracy of these conventional speed sensing systems depend heavily, among other factors, on the accuracy of the spacing between the teeth in a toothed target, the spacing of the magnetic poles in a magnetic target, and the spacing of the bars in a bar encoder. Thus, for a precision system, a target with high spacing accuracy is preferred.
However, the target manufacturing cost is proportional to the target spacing accuracy requirements, and it is not always economical to construct a large outer diameter angular target wheel or a long linear target with high spacing accuracy. Accordingly, it would be advantageous to introduce a speed sensing system which maintains a high degree of speed measurement accuracy without requiring the production and application of a precision speed sensing target.